This invention relates to a four-stroke cycle engine unit for a portable working machine and more particularly to a crank chamber of an improved structure of a four-stroke cycle engine unit provided with a cantilevered crank shaft.
Usually, a portable working machine such as a trimmer or lawn mower is equipped with an internal engine unit which is not always constant in its posture in the driving state, and it is often driven with an inclination of 40.degree. to 50.degree. in back and forth or lateral direction. Moreover, since such a working machine is usually handled by holding the same by hands of an operator, so that it is required for the engine unit to have a light weight and operate at high revolution speed. In order to satisfy such requirements, it may be desired to incorporate a two-stroke cycle engine to the portable working machine rather than a four-stroke cycle engine.
However, the four-stroke cycle engine provides the following advantages in comparison with the two-stroke cycle engine. That is, the four-stroke cycle engine generates relatively low noises, exhausts relatively clean exhaust gas, and relatively less amount of fuel is consumed in comparison with the two-stroke cycle engine. In consideration of these advantageous points, there has a tendency to incorporate the four-stroke cycle engine to a portable working machine in actual use.
However, when the four-stroke cycle engine is utilized in such inclined state or posture, lubrication oil fed into a crank chamber of the engine unit is excessively agitated by a counter-weight member provided for the crank shaft, which will result in adverse heat generation and causes or undesired resistance, oil-up state, etc.
Furthermore, during the engine operation in the inclined state, there is a problem that a magnitude of a pulsation pressure of a gas, almost air, in the crank chamber is insufficient for driving a fuel pump in association with a carburetor, i.e. membrane valve type carburetor. Namely, in the case of the four-stroke cycle engine unit, since a crank shaft, a cam shaft, a lubrication oil reservoir and the like are accommodated in the crank chamber, the space volume in the crank chamber is made large in comparison with the two-stroke cycle engine unit. Accordingly, a pulsation pressure of the gas having sufficient magnitude cannot be obtained by the reciprocal motion of a piston of a piston-cylinder assembly of the engine unit, thus being not suitable for the operation of the membrane valve type carburetor.